One of the main applications of the actuation systems to which the present invention pertains relates to mechanisms of “Phase Modulation” type for optical instruments in space, intended to perform rotational or translational cycles of a “modulator plate” with position increments (four-phase modulation).
In an optical instrument, of interferometer type, this mechanism is situated between the splitter plate and one of the two stepped mirrors. The angle of “tilt” (case of rotation) or the linear displacement (case of translation) of the “modulator plate(s)” makes it possible to modify the length of an optical path.
In this type of mechanism, the technical problem resides mainly in several points:                The loads and moments must be balanced and must remain internal to the mechanism, so as not to disturb the remainder of the instrument.        In the case of several “modulator plates”, their respective motions must be perfectly synchronized, while limiting the number of actuators (in the case of a single optical plate, a “counterweight” element is used).        The assembly, when it is aboard a spacecraft, during the launch phase, must be “integral”. This means that, despite the absence of any specific lashing system, it must not degrade under the effect of the accelerations generated by the launcher.        
Shown diagrammatically in FIG. 1 is an autonomous and non-disturbing actuation system 1 for controlling opposite and synchronized rotational motions of two elements, an optical plate 11, and a counterweight frame 10 which are supported in rotation about a common axis “O” with the aid of a bearing comprising the flexible elements 12, 13 and which is similar to the bearing 34 represented in FIG. 5, the assembly forming part of an optical space instrument. This system essentially comprises: a support frame 9, two actuators 14a, 14b disposed so as to exert a couple between the elements 10 and 11, so as to orient them one with respect of an angle α. The actuators are of piezo-electric type. The element 10 comprises at its ends inertia pieces 10a, 10b intended to limit the mass of the assembly. The limits of the path of the optical beam received by the device 1 have been delimited by two dashed lines T1, T2.
The system represented in FIG. 2, of the type with linear motions, comprises: a carrier rigid structure 9 supporting two compensating prismatic optical plates 10 and 11 by way of identical flexible metallic guidance platelets 12a, 13a, these two compensating prismatic optical plates 10 and 11 being propelled by two specific actuators 14a and 14b. A plate 13a is represented in the magnified detail view in the right part of FIG. 2.
During the operation of a linear mechanism of this type, in order to limit the forces tending to disturb the instrument, each setting of an element into motion must be compensated by an equivalent load in the opposite direction and along an axis passing through the centres of gravity of the elements in motion. The same holds for rotary mechanisms where the centres of gravity of the elements in motion must preferably be situated on a single axis of rotation.
The system presented in FIG. 1 proposes a specific actuator for motorizing each element, thereby multiplying the number of components. Synchronization of the motions is obtained by complex electronic circuits. The non-convergence of the thrust vectors of the actuators and the desynchronization of the motions generate dispersions towards the instrument.
The linear system presented in FIG. 2 also proposes a specific actuator for motorizing each of the two elements. The non-convergence of the two thrust vectors of the actuators and the electronic synchronization of the motions generate dispersions towards the instrument.